Conventional high speed, subsonic airplanes are designed with large stabilizer surfaces and a forward center of gravity in the range of 25%-35% mean aerodynamic cord (MAC) for normal stability. Both the large stabilizer surface and the forward center of gravity of such airplane decrease aircraft performance and increase fuel usage. Some commercially operational airplanes have a 12% MAC forward center of gravity limit and a 35% MAC aft center of gravity limit as compared with the conventional baseline airplane with a center of gravity location at 25% MAC.
The economics of increased performance with lower fuel use becomes a compelling incentive for improved aerodynamic performance of all airplanes. An optimum, new aircraft design must be capable of high aerodynamic performance with low fuel usage at any angle of attack. It must have reduced aerodynamic trim drag and high maneuverability. It must have lower tail drag and lower structural weight. Reduced aerodynamic trim drag can be achieved by a further aft center of gravity balancing. The tail drag can be reduced by the use of a smaller, horizontal tail surface or a new tail wing design. The further aft center of gravity feature coupled with a new tail section design can also aid in the reduction of tail drag.
Increased aircraft performance can also be achieved by the use of supercritical wings which increase significantly the lift to drag ratio (L/D) at conventional static margins. However, the trim drag is very large. Supercritical wings at normal or conventional static margins (25% MAC) produce twice as much trim drag as conventional wings at conventional static margins. Thus, for supercritical wings at normal static margins, to eliminate the trim drag, the airplane would have to fly 10%-15% statically unstable. To do so would require the use of a stability and control augmentation system or pitch active control system (PACS). Such a system permits an aircraft to operate at reduced static margins approaching the neutral point.
Research has shown that with PACS non-operational, the flying qualities degrade quickly as the center of gravity moves aft and approaches the static neutral point (40% MAC) and becomes unacceptable for negative static margins of 5% or less, i.e., 40%-45% MAC. With the center of gravity at 35%-45% MAC, the airplane is still flyable. At 45%-50% MAC, the airplane becomes essentially unflyable.
For optimum performance, both the supercritical wings and a further aft center of gravity with balance is used. This combination produces an even higher L/D ratio without the large trim drag; however, the airplane would have to operate at 10%-15% static instability with the use of an effective stability and automatic control system.
Some of the desired features of aft center of gravity airplanes with controlled balance, i.e., PACS activated, are as follows:
(1) They can fly higher because the center of pressure is ahead of the center of gravity;
(2) With control balance, they can improve airport efficiency and performance and (a) allow slower takeoff speeds, (b) allow slower landing speeds, and (c) make use of shorter landing strips;
(3) As the center of gravity moves aft to 50% MAC, they have satisfactory handling qualities; however, degradation will occur with the center of gravity at 50%-60% MAC. The handling qualities with the center of gravity at 50% MAC (10% statically unstable) are as good as the basic unaugmented aircraft with the center of gravity at 25% MAC (15% statically stable);
(4) They have stall attitudes which become higher as the center of gravity moves aft.